Process for the destruction of halocarbons

ABSTRACT

PCT No. PCT/GB93/01656 Sec. 371 Date Mar. 30, 1995 Sec. 102(e) Date Mar. 30, 1995 PCT Filed Aug. 5, 1993 PCT Pub. No. WO94/03237 PCT Pub. Date Feb. 17, 1994A process for the destruction of a halocarbon, which process comprises reacting the halocarbon with molten sodium at an elevated temperature to produce the corresponding sodium halide or halides in a sludge in the molten sodium and separating the sludge from the molten sodium.

The present invention relates to a process for the destruction ofhalocarbons.

World wide concern over the depletion of ozone in the upper atmospherehas led to stringent international measures covering the production anduse of halocarbons, particularly chlorofluorocarbons (CFCs) used inrefrigeration, foam blowing, aerosols, etc. Intense work in thedeveloped world is being targetted towards the search for some "ozonefriendly" alternative compounds for use in these applications. However,considerable quantities of CFCs are presently "locked-up" in existingequipment. As this equipment comes to the end of its useful life therewould be great value in being able to extract the CFCs and destroy thematerial in a process which would remove the halocarbons from theatmosphere.

We have now developed a process in which halocarbons are converted frommaterials with a high ozone depletion potential into stable solid phasecompounds.

Accordingly, the present invention provides a process for thedestruction of a halocarbon, which process comprises reacting thehalocarbon with molten sodium at an elevated temperature to produce theappropriate sodium halide or halides in the solid phase.

The reaction of the present invention is surprising since it would havebeen predicted that a very violent, if not explosive reaction, wouldoccur when the halocarbon contacted the molten sodium.

The process of the present invention, although specifically designed toaddress the chlorofluorocarbon problem, could also be used to treat awide range of other halocarbons, such as dry cleaning fluids, degreasingfluids, polychlorobiphenyls, etc.

In carrying out the process of the present invention, the molten sodiumis maintained at a temperature in the range of from 98° to 500° C.,preferably at a temperature in the range of from 200° to 300° C.

The halocarbon which is to be destroyed in the present invention may bebubbled through the molten sodium, or may be admixed with an inert gas,such as, nitrogen, helium or argon.

The halocarbon to be treated according to the process of the presentinvention is preferably bubbled from a reservoir through the moltensodium, for example by introducing the gas into the bottom of a reactionvessel containing molten sodium.

The halocarbons react with the molten sodium to produce solid productseach of which is denser than molten sodium. The products therefore tendto sink to the base of the reactor from which they are preferablyremoved, unless the reaction mixture is stirred vigorously when theproducts will tend to stay on top of the molten sodium. The densitiesand melting points of the various species involved are given below.

    ______________________________________                                                     Density                                                                              Melting Point °C.                                  ______________________________________                                        Sodium         0.97     97.8                                                  Sodium Bromide 3.203    747                                                   Sodium Chloride                                                                              2.165    801                                                   Sodium Floride 2.557    993                                                   Carbon         1.8-2.2  3600                                                  ______________________________________                                    

A typical process is, for example, the destruction ofdichlorodifluoromethane (R12), a typical domestic refrigerant, whichreacts according to the following equation:

    CCl.sub.2 F.sub.2 +4Na→C+2NaCl+2NaF.

The products of the process of the present invention are solid, dryproducts which are easily separable from the molten sodium. The productsare, furthermore, valuable e.g. the carbon produced is in finely dividedform with a very high surface area and is useful as a catalyst carrieror absorbent, etc. Should the treated halocarbon contain a bromine atomthe resultant bromide products so produced may be readily converted intoelemental bromine. Because no aqueous products are formed in thereaction, the process is particularly suitable for use as anenvironmentally friendly method for the destruction of ozone depletingvolatile halocarbons.

The present invention will be described by way of a specific examplethereof with reference to the single Figure of the accompanying drawingswhich illustrates an appartus for carrying out the process.

Referring to the drawing, a reaction vessel 1 is filled with moltensodium 2. The molten sodium is held at a temperature above the meltingpoint thereof, i.e. above 98° C. by means of a coil heater 3 whichsurrounds the column of the reaction vessel. The halocarbon which is tobe treated according to the invention is contained in a reservoir 4which is cooled by means of a cooler 5 through which a coolant flows inorder to maintain the halocarbon liquid. The reservoir 4 is providedwith a tap 6 for the introduction of a further supply of halocarbonthereto. An inert gas is pumped by means of pump 7 along line 8 into thehalocarbon reservoir. The bottom of line 8 is immersed in the liquidhalocarbon and as the inert gas bubbles from the tube the halogencompound is vapourized. The concentration of the halocarbon which ismixed with the inert gas stream will depend upon the vapour pressure ofthe halocarbon and the flow rate of the inert gas stream along line 8.The mixture of halocarbon and inert gas passes along line 9 via a oneway valve 10 to the reactor 1. Line 9 enters reactor 1 close to thebottom thereof and the mixture of inert gas/halocarbon bubbles throughthe molten sodium during which time the halocarbon reacts with themolten sodium in order to form sodium halides and carbon. The sodiumhalides and carbon produced according to the process settled to thebottom of the reactor as a sludge 11. Periodically the sludge 11 will beremoved from reactor 1 by opening tap 12. After passage through themolten sodium 2 the inert gas, essentially freed from the halocarbonvapour, leaves reactor 1 via a loop line 13 which joins line 8 via pump7. Periodically, it will be necessary to top up the level of sodiumcontained in reactor 1 and the sodium may be introduced into the top ofthe reactor through an inlet port 14.

The present invention will be further described with reference to thefollowing Examples.

EXAMPLE 1 Reaction of R-12 (CF₂ Cl₂) with Molten Sodium

A 500 cm³ 4-necked flanged flask equipped with a gas inlet, sealedstirrer unit, thermometer and an outlet open to the atmosphere (via twospiral cold glass traps, two oil bubblers and a silica gel moisturetrap) was charged with sodium metal (123.05 g, 5.35 g-atom). The flaskwas immersed in an oil bath and heated to 146° C. The R-12 gas (CF₂ Cl₂)was bled into the stirred molten sodium via a calibrated flowmeter sothat the amount could be regulated and monitored at any given time, andany escape of the gas could be noticed by watching the oil bubbler.

The R-12 flowrate was initially at 8 cm³ /min for 1 hr; after this timea black layer appeared on the top of the silvery metal (R-12=1.65 g,13.6 mmol; 76 mins). The flowrate was then increased to 14 cm³ /min for1 hr, during which period although occasional bubbles in the oil-filledtrap were noticed from time to time, no material condensed in the coldtraps. The amount of the black powdery layer (in addition to otherlighter-coloured solids) kept forming on the top of the molten metal(R-12=5.15 g, 42.6 mmol; 60 mins). Finally, the flow-rate was increasedto 30 cm³ /min for 20 mins; this caused the rate of production of thebubbles in the oil-filled lute to be increased to 120/min in comparisonto 4-10/min in the previous stage. The flowrate was so great that someof the carbon powder produced was swept into the traps; and theunreacted R-12 escaped uncondensed (R-12=2.30 g, 19 mmol; 20 mins).

    ______________________________________                                        The Mass Balance                                                              Input       (g)            Output     (g)                                     ______________________________________                                        T.   reaction flask                                                                           939.95                                                        G.   reaction flask                                                                           1063.00  G.  reaction flask                                                                           1070.60                               N.   Sodium metal                                                                             123.05                                                             R-12 Stage 1                                                                             1.65                                                               R-12 Stage 2                                                                             5.15                                                               R-12 Stage 3                                                                             2.30                                                               R-12 total 9.10         mass increased                                                                           7.60                                                               84% conversion                                   ______________________________________                                    

The reaction was slightly exothermic due to the heat exchange with theoil bath.

EXAMPLE 2 Reaction of 1301 CF₃ Br) with Molten Sodium

A 1000 cm³ 5-necked flanged flask equipped with a gas inlet, sealedstirrer unit, thermometer and an outlet open to the atmosphere (via twooil bubblers) was charged with sodium metal (189.50 g, 8.24 g-atom). Theflask was immersed in an oil bath and heated to 121° C. The 1301 gas(CF₃ Br) was bled into the stirred molten sodium via a calibratedflowmeter so that the amount could be regulated and monitored at anygiven time, and any escape of the gas could be noticed by watching theoil bubbler.

The flowrate of the 1301 was increased gradually and no diluent wasemployed. Firstly, the gas 1301 was admitted at a rate of 5 cm³ /min;after approximately 30 mins a black layer appeared on the top of thesilvery metal. After a total of 40 mins the flowrate was increased to 8cm³ /min for 30 mins, during which period the amount of the blackpowdery layer (in addition to other lighter-coloured solids) continuedto form on the top of the molten metal. The flowrate was then increasedto 20 cm³ /min for 30 mins, subsequently to 33 cm³ /min (30 mins.), to48 cm³ /min (10 mins) and finally to 55 cm³ /min (10 mins). During thewhole time the 1301 gas reacted completely with the molten sodium and noappearance of the gas in the exit line was noticed at any time. Thereaction was slightly exothermic. The stirring was vigorous andeffective in such a way that the upper part of the anchor stirrer wasacting as a centrifuge towards the solid products which accumulated ontop of the molten sodium; a certain amount of solid product was observedto sink near the walls of the glass reactor.

    ______________________________________                                        Mass Balance                                                                  Input         (g)     Output         (g)                                      ______________________________________                                        Tare wt. reaction flask                                                                     1011.85                                                         Gross wt. reaction flask                                                                    1201.34 Gross wt. reaction                                                                           1217.45                                  Net wt. Sodium metal                                                                        189.50  flask                                                   13B1 total    16.16   mass increase  16.11                                                          99.8% conversion of                                                           CF.sub.3 Br to products                                 ______________________________________                                    

We claim:
 1. A process for the destruction of a halocarbon, whichprocess comprises reacting the halocarbon with sodium characterised inthat the halocarbon in gaseous form is bubbled through a bath formed ofmolten sodium at a temperature in the range from 98° to 500° C. toproduce the corresponding sodium halide or halides in a sludge in themolten sodium and separating the sludge from the molten sodium.
 2. Aprocess as claimed in claim 1 wherein the halocarbon is achlorofluorocarbon.
 3. A process as claimed in claim 1 wherein themolten sodium is maintained at a temperature in the range of from 200°to 300° C.
 4. A process as claimed in claim 1 wherein the halocarbon isadmixed with an inert gas.
 5. A process as claimed in claim 4 whereinthe halocarbon is carried from a reservoir in a stream of inert gas andthe mixture of halocarbon/inert gas is bubbled through the moltensodium.
 6. A process as claimed in claim 4 wherein the halocarbon or themixture of halocarbon/inert gas is introduced into the bottom of areaction vessel containing the molten sodium.
 7. A process as claimed inclaim 1 wherein the solid phase products produced in the reactionmixture are separated from the molten sodium.
 8. A process as claimed inclaim 1, said process consisting essentially of reacting halocarbon withsodium wherein said halocarbon in gaseous form is bubbled through a bathformed of molten sodium at a temperature in the range from 98° to 500°C. to produce the corresponding sodium halide or halides in solid phase.